This invention relates to methods of controlling the brightness of a glow discharge. The methods relate particularly, though not exclusively, to light sources for backlighting liquid crystal displays.
Glow discharge light sources are increasingly being used as backlights for liquid crystal displays. Such backlights must be capable of high brightness for use in direct sunlight, and have applications in vehicle instrument displays, aircraft cockpits etc. When such displays are used in low light conditions, or when the observer is wearing image intensifying goggles to improve night vision, such high source brightness becomes a disadvantage. For this reason a number of methods of dimming LCD backlights have been developed.
One method of controlling the brightness of a glow discharge light source is to use a train of excitation pulses and to modify the duration of the pulses. This is known as pulse duration modulation, and the brightness of the light source can be reduced in proportion with the average power supplied to the lamp. There are, however, a number of drawbacks with such techniques. In U.S. Pat. No. 5,349,273 for example it is disclosed that only a 20:1 dimming range is possible because of significant illumination non-uniformity at low lamp currents, and because of a reduction in output voltage of the controller resulting in non-excitation of the discharge. Most commercially available fluorescent lamp dimmers have a dimming range of less than 150 to 1.
In U.S. Pat. No. 5,420,481 a supplementary set of electrodes are used to operate a glow discharge in a different manner in a low brightness regime. By switching from one set of electrodes to the other set it is possible to achieve a dimming range approaching 10,000:1 (or 80 dB) from 3000 cd m.sup.-2 to 0.3 cd m.sup.-2. However the maximum brightness of this lamp is not high enough for good contrast displays in bright sunlight, and the provision of extra electrodes and switching circuitry increases cost and decreases reliability and convenience of use. There can also be a discontinuous change in brightness when switching from one set of electrodes to the other set.
According to a first aspect of the invention there is provided a method of controlling the brightness of a discharge capable of operating in a first condition having a first brightness and in a further condition having a different brightness, the said conditions occurring in adjacent time periods, the method comprising
a) supplying r.f. energy to the discharge as a train of pulses, and PA1 b) controlling the duration of the pulses, thereby controlling the ratio of the time spent by the discharge in the first condition to the time spent by the discharge in the further condition in any given time period, such that any change in the duty factor of the train of pulses is proportionally less than the resulting change in brightness of the discharge. PA1 a) supplying r.f. energy to the discharge as a plurality of sets of pulses, each set having a different pulse duration, at least one set having a pulse duration sufficiently short that the discharge is in the said first condition for the whole duration of each pulse in the said at least one set, and at least are further set having a further pulse duration sufficiently long that the discharge passes into both conditions during each pulse in the said at least one further set, and PA1 b) controlling the repetition rate of the pulses comprising the at least one further set of pulses, thereby controlling the ratio of the time spent by the discharge in the first condition to the time spent by the discharge in the second condition in any given time period. PA1 a) supplying r.f. energy to the discharge as a plurality of sets of pulses, each set having a respective pulse duration, at least one set having a pulse duration sufficiently short that the discharge is in the said first condition for the whole duration of each pulse in the said at least one set, and PA1 b) controlling the duration of the pulses in each of the sets of pulses in synchrony with the other sets.
This method can provide brightness control which is continuously variable over a brightness range in excess of other known methods, the brightness range being surprisingly greater than the range of duty factor variation.
Preferably the method is such that in the first condition r.f. energy is mainly electric field coupled to the discharge and in the further condition r.f. energy is mainly magnetic field coupled to the discharge. The r.f. energy is advantageously mainly electric field coupled to the discharge at the start of a given pulse.
According to a second aspect of the invention, there is provided a method of controlling the brightness of a glow discharge capable of operating in a first condition having a first brightness and in a further condition having a different brightness, the said conditions occurring in adjacent time periods, the method comprising
This method can provide a plurality of brightness levels which are less susceptible to temperature variations and other variables which are difficult to control.
According to a third aspect of the invention, there is provided a method of controlling the brightness of a glow discharge capable of operating in a first condition having a first brightness and in a further condition having a different brightness, the said conditions occurring in adjacent time periods, the method comprising
This method can also provide a plurality of brightness levels which are less susceptible to temperature variations and other variations which are difficult to control.